Direct sequence code division multiple access (DS-CDMA) allows signals to overlap in both time and frequency so that CDMA signals from multiple users simultaneously operate in the same frequency band or spectrum. In principle, a source information digital data stream to be transmitted is impressed upon a much higher rate data stream generated by a pseudo-random noise (PN) code generator. This combining of a higher bit rate code signal with a lower bit rate data information stream “spreads” the bandwidth of the information data stream. Each information data stream is allocated a unique PN or spreading code (or a PN code having a unique offset in time) to produce a signal that can be separately received at a receiving station. From a received composite signal of multiple, differently-coded signals, a PN coded information signal is isolated and demodulated by correlating the composite signal with the specific PN spreading code associated with that PN coded information signal. This inverse, de-spreading operation “compresses” the received signal to permit recover of the original data signal and at the same time suppresses interference from other users.
Space diversity (sometimes called macrodiversity) is attained by providing multiple signal paths through simultaneous links from a mobile station to two or more base stations. When the mobile station is in communication with two or more base stations, a single signal for the mobile user is created from the signals from each base station. This diversity communication is sometimes referred to as “soft” handover in that communication with a destination base station is established before communication with the source base station is terminated, i.e., a make-before-break type of handover. Thus, after a call is initiated and established between a mobile station and a source base station, the mobile station continues to scan a broadcast signal transmitted by base stations located in neighboring cells. Broadcast signal scanning continues in order to determine if one of the neighboring base station transmitted signals is strong enough for a handover to be initiated. If so, this determination is provided to the radio network which sends the appropriate information to the mobile station and to the new destination base station to initiate the diversity handover. The new base station searches for and finds the mobile station's transmitted signal using the associated spreading code. The destination base station also begins transmitting a downlink signal to the mobile station using the appropriate spreading code. The mobile station searches for this downlink signal and sends a confirmation when it has been received.
In each cell, the base station selects the strongest paths for demodulation. The demodulated information from each of these strongest paths are combined using, for example, some form of maximal ratio combining. In addition, the radio network combines the two (or more) versions of the mobile station uplink signal from the base stations involved in a diversity soft handover, and either selects the signal with the best quality or combines the signals to achieve an optimal signal. The result of these various combining and selecting operations is a greatly improved resistance to fading and other adverse influences often encountered in mobile radio communications.
Diversity handover requires timing synchronization between the source and destination base stations and the mobile station. All nodes involved in a macrodiversity handover should be synchronized on a frame level so that the same frame is sent from all base stations involved in the handover to the mobile station at the same time. Frame level synchronization between the diversity handover base stations may be accomplished using a system frame number (SFN) counter in each of the nodes or using some other common internal clocking mechanism. In this description, the diversity handover nodes include a radio network controller (RNC), a source radio base station, a destination radio base station, and a mobile station. The system frame number counters in each of these nodes must be synchronized within some relatively small deviation. The RNC may periodically perform phase measurements to determine any deviation between the value in its system frame number counter and the value in each of the source and destination base station system frame number counters. If the deviation is out of tolerance, the RNC may order one or both of the source and destination base stations to adjust its respective system frame number counter. In order to avoid losing entire frames or symbols, this adjustment is preferably performed in small incremental steps such as one-eighth of a chip or some other multiple of the internal clock. A smaller increment reduces the degree to which the received signal is degraded.
The mobile station also adjusts its timing in order to be synchronized to the RNC and source and destination base stations. Such adjustment is typically a necessary and continuing process due to the mobility of the mobile station. Typically, the mobile station locks its timing to that of the source base station by monitoring timing information included in the source base station's broadcast channel and adjusting the mobile's internal clock accordingly in small increments. Accordingly, during a macrodiversity handover, the source and destination base stations as well as the mobile station may often be adjusting their respective timing mechanisms at the same time. As a result, there is a risk that as the mobile station adjusts its timing in one direction, e.g., forward, the base station adjusts its timing in the opposite direction, e.g., backward. Thus, the total timing adjustment and its degrading effect on the quality of the received signal at both the base station and the mobile station significantly increase when the timing adjustments move in opposite directions. For example, if the mobile station adjusts its timing by one-eighth of a chip duration in one direction and the radio base station one-eighth of a chip duration in the opposite direction, the total timing adjustment in the received signal is effectively one-fourth of a chip duration.
The present invention avoids this degradation in signal quality by ensuring that a base station and a mobile station do not adjust their timing simultaneously. Generally, a timer or a clock in each of the mobile and base stations is not adjusted at the same time. For example, the timing of the base station may be changed during a first time interval while that of the mobile station may be changed during a second different time interval. A radio network controller determines the difference between the base station timing and the timing of the radio network. If that difference exceeds a threshold, the radio network controller determines a timing adjustment based on the difference. The timing adjustment is communicated to the base station which incrementally adjusts its timing during the first set of time intervals. The mobile station detects the base station timing and adjusts its own timing during a second set of time intervals. In one example, non-limiting embodiment, the base station may adjust its timing only at odd time intervals, such as odd system frame numbers, and the mobile station may adjust its timing only at even time intervals, such as even system frame numbers (or vice versa).